When injection molding plastic preforms, as for example preforms of polyethylene terephthalate (PET), some systems form a gate nub or projection area from the molded part that is allowed to solidify on the molded part and is later removed from the molded part in a subsequent operation. This gate nub or projection is a depository for undesirable resin crystallinity that typically forms in the mold gate area. If the nub is subsequently cut off one has a molded preform which is free of gate crystallinity. Such a system is shown in U.S. Pat. NO. 4,588,370 to Ichizawa et al., patented May 13, 1986.
A second approach is to cause the gate nub to break off the molded part during mold opening and then eject the removed nub from the gate area when the mold is open. This is shown in Japanese Patent Application No. 52-151358, published Dec. 15, 1977. In this procedure the valve stem of a hot runner has a "gate puller" undercut machined into its end so that the gate nub solidifies and forms around the gate puller when the valve stem is in the gate closed position. Then as the mold opens the valve stem is retracted to break off the nub which then appears to fall free of the mold when the mold cavity is moved away from the hot runner system. This considerably complicates mold construction and does not appear to provide a foolproof nub removal and ejection system.
The automatic degating of a sprue is known in other molding applications. U.S. Pat. NO. 4,820,467 to Ehrler et al., patented Apr. 11, 1989, shows a system in which a hot runner molds a cold sprue gated disc. After solidification of the sprue a sleeve surrounding the nozzle tip advances to eject the sprue and degate it from the molded part and simultaneously form a hole therein. The cold, degated sprue is then automatically conveyed away from the gate area down a chute in the mold.
U.S. Pat. NO. 5,346,659 to Buhler et al., patented Sep. 13, 1994, shows another cold sprue gating and ejection system similar to the '467 patent in which the ejection means is built into the top of the mold core.
U.S. Pat. NO. 5,423,672 to Gordon, patented Jun. 13, 1995, shows a molding device for forming a disc with a hole therein. This patent shows a valve gated hot runner in which the valve stem is moved to an intermediate position by means of a dual piston combination. In the disc molding operation the valve stem is first moved forward to open the valve gate and allow resin to fill the mold cavity. Next the valve stem is partially retracted to block the melt flow and allow a lower part of the valve stem to form the hole in the disc. Finally, the valve stem is fully retracted to pull its hole forming section out of the molded part and to allow the molded part to be ejected conventionally from the core side of the mold.
U.S. Pat. NO. 3,671,159 to Greenberg et al., patented Mar. 6, 1970, shows a valve gating system in which the valve stem is hollow and conveys compressed air to assist in ejecting the part as the mold is opened. The valve stem is advanced into the mold cavity to expose the air channel orifice in the valve stem and to allow it to assist in the mold opening and part ejection. A system such as this, however, risks blocking the air passages during injection of the melt while the mold cavity is being filled.
It is a principal object of the present invention to provide an improved injection nozzle system and method for injection molding which includes a movable valve stem and a valve gate nub area.
It is a further object of the present invention to provide a device and method as aforesaid in which the valve stem is conveniently and expeditiously used to advance into the mold cavity-valve gate nub area in order to aid in ejection of the molded part and to clear debris from the valve gate area.
Further objects and advantages of the present invention will appear hereinbelow.